Femtosecond structural photobiology

Absorption of a photon in the visible region of the solar spectrum is the fundamental step in the photochemical reactions that drive biological processes such as imaging, photosynthesis, and sensory perception. All known photoreceptors are proteins that contain a chromophore: a small organic molecule that absorbs in the visible region of the spectrum ( 1 ). The bacterial integral membrane protein bacteriorhodopsin (bR) uses retinal as its chromophore (see the figure), capturing the energy in a photon to drive retinal isomerization and pump protons across the membrane. bR is a readily accessible analog to the visual rhodopsins in, for example, the human eye. On page 145 of this issue, Nogly et al. ( 2 ) used time-resolved x-ray crystallography ( 3 , 4 ) to determine intermediate structures of bR at near-atomic resolution on the femtosecond to picosecond time scales at which isomerization occurs.